1. Technical Field
The present invention relates to a high electron mobility transistor (HEMT) of a normally-off type (enhancement type) formed of a nitride semiconductor device.
2. Description of the Related Art
A nitride semiconductor has an electric field drift speed and a breakdown electric field strength very higher than Si or GaAs. At a heterojunction interface occurring between nitride semiconductors, a high-density carrier layer called a two-dimensional carrier gas is formed by a polarization effect. From these features, for example, a high electron mobility transistor (HEMT) having an AlGaN/GaN heterojunction structure is applied as a switching device for power source operating at a large current and a high voltage.
The HEMT using GaN (0001) plane has a negative gate threshold voltage, and has characteristics in which a current flows between a source electrode and a drain electrode in a state in which a gate control voltage is not applied to a gate electrode, that is, characteristics of a normally-on type (depletion type). However, a switching device is desired to be a normally-off type (enhancement type) having a positive gate threshold voltage in order to ensure safety at the time of abnormality.
JP-A-2007-165719 describes a HEMT in which a channel layer made of GaN is formed on a barrier layer made of AlGaN in a GaN/AlGaN heterojunction. According to the HEMT, a lattice constant of the channel layer is larger than a lattice constant of the barrier layer, so that piezo polarization and spontaneous polarization occurring in the channel layer are canceled and thereby a carrier concentration of a two-dimensional electron gas (2DEG) formed in the channel layer can be reduced and the HEMT having characteristics of a normally-off type can be obtained.